Cemented carbide compositions and process for producing the same

ABSTRACT

Cemented tungsten carbide compositions exhibiting high bending strength (transverse rupture strength) combined with high hardness are provided by using specially prepared tungsten carbide produced from tungsten powder formed by the hydrogen reduction of tungsten chloride.

United States Patent Ramovist 1 Jan. 30, 1973 [54] CEMENTED CARBIDE COMPOSITIONS [56] References Cited AANlggRocEss FOR PRODUCING THE UNITED STATES PATENTS inventor: Lars Henry Ramovist, Nynashamn, 3,539,335 11/1970 Svanstriim et al. ..75/84.5 X

Swede" OTHER PUBLICATIONS Assigneei Rederiakfiebolaget Nordstjel'nam, Schwarzkopf et al., Refractory Hard Metals, MacMil- Nynashamn, Sweden I n, l953 pp. 49-56.

22 F] d: D .4 1970 l 1 l 6 cc Primary Examiner-Carl D. Quarforth [21] Appl. N .2 95,104 Assistant Examiner-R. E. Schafer Attorney-Sandoe, Hopgood and Calimafde [30] Foreign Application Priority Data ABSTRACT April 2, 1970 Sweden ..4530/70 Cemented tungsten carbide compositions exhibiting high bending strength (transverse rupture strength) [52] U.S.Cl ..29/l82.8,75/.5 BB,75/84.5, combined with high hardness are provided by using [5 t C 2 f 1 /:N specially prepared tungsten carbide produced from O u I 1 e I t t d f d b h d d t f [58] Field of Search ..75/s4.5,.5 BB, 204,211; ungse" er y e y mgenre tungsten chloride.

7 Claims, No Drawings CEMENTED CARBIDE COMPOSITIONS AND PROCESS FOR PRODUCING THE SAME This invention relates to cemented carbide compositions, such as tungsten carbide, having an improved combination of high bending strength coupled with high hardness and to a process for producing such compositions.

STATE OF THE ART Owing to recent developments in modern technology, increasing requirements have been placed on the use of construction materials, such as high speed steels, improved high strength alloys and the like. Consequently, considerable burden has been placed on the materials used for cutting, working and forming such construction materials. The research and development work in the field of cemented carbides, therefore, has been directed towards increasing the values of such physical properties as hardness, transverse rupture strength, and the like, in the carbide systems used. However, since it is well known that an increase in hardness generally implies a decrease in transverse rupture strength, it generally has been the practice to accept a compromise between these two properties.

With regard to cemented carbide compositions well known in the art, it is not uncommon to obtain hardness values of up to 1,800 kg/mm (H, 330 kg load) and transverse rupture strength of about 150 kglmm whereas, with hardness values in the neighborhood of about 1,000 kg/mm, the transverse rupture strength may range up to 300 kglmm In the production of cemented carbide compositions, it has been known for at least several decades that low transverse rupture strength values are generally caused by faults in the cemented carbide structure, a primary source of trouble being voids in the cemented carbide. It has further been known that contaminated refractory carbides tend to give rise to a greater number of voids during the sintering process than when working with pure refractory carbides, particularly if the carbide contains oxides, such as silicon dioxide, which, together with carbon, form gaseous products at an elevated temperature. Additionally, it has been discovered that carbide grains of too large a size, in non-homogeneous structures, act as crack formers with the result that low transverse rupture strength values are obtained.

The purity of conventionally produced tungsten carbide can be substantially improved to provide good transverse rupture strength values; however, the method employed requires taking cost increasing additional steps in the carburizing process, for example, by subjecting the tungsten carbide after the production in hydrogen gas to an after-treatment at a high temperature, possibly in combination with vacuum. It is possible by this method to achieve values of up to 350 kg/mm for coarse-grained cemented carbide alloys with about percent binding phase (e.g. cobalt) in combination with a hardness of between about 1,200 and 1,250 kg/mm However, the application of such special steps in the carburizing process involves certain disadvantages, in addition to the economical one mentioned hereinabove. For example, difficulties tend to arise as a result of crushing the carbide cakes in preparation for sintering such that during sintering the carbides generally undergo grain growth which, besides being non-uniform, tends to result in a non-uniform microstructure which causes a deterioration in the transverse rupture strength and hardness values of the final cemented carbide product.

Conventional cemented carbides, based on tungsten carbide powder produced from tungsten obtained by the reduction of tungsten trioxide, tends to exhibit a non-uniform microstructure, owing to the fact that the grain growth of the carbide is largely affected by the sintering temperature. A high sintering temperature generally gives rise to substantial grain growth, whereby the resulting composition exhibits low transverse rupture strength. The solution to this problem is to control accurately the sintering temperature. Moreover, the design of the furnace equipment should be such as to assure only small temperature gradients. When using batch furnaces, for example, in most cases only a part of the furnace space can be utilized. As is apparent, the foregoing clearly implies practical disadvantages.

It is known to produce tungsten powder by the plasma hydrogen reduction of tungsten hexachloride (WCl However, such powders are not of any interest due to the extremely fine particle sizes obtained, e.g. substantially less than 0.1 micron.

OBJECTS OF THE INVENTION It is thus the object of this invention to provide cemented tungsten carbide compositions characterized by improved combination of physical properties, such as high transverse rupture strength combined with high hardness.

Another object is to provide a process for producing cemented tungsten carbide compositions exhibiting improved high transverse rupture strength combined with high hardness.

These and other objects will more clearly appear from the following disclosure and the appended claims.

STATEMENT OF INVENTION Broadly speaking, one embodiment of the invention resides in a process of producing a cemented carbide composition in which the tungsten carbide employed in the composition is formed by carburizing tungsten powder produced by the hydrogen reduction of gaseous tungsten chloride. A preferred method particularly applicable for producing tungsten powder from which high quality tungsten carbide has been produced is disclosed in US. copending application Ser. No. 35,991, filed May 11, 1970. Broadly, the method disclosed for producing the tungsten powder comprises providing a preheated substantially unreacted mixture of the reactants tungsten hexachloride and hydrogen maintained at a temperature above the reaction temperature for said reactants, and then immediately feeding the preheated mixture into a reaction chamber maintained at at least above the reaction temperature of the reactants, whereby the reaction to tungsten metal powder is substantially spontaneously effected in the reaction chamber. The tungsten metal powder is then carburized to tungsten powder in the known manner.

The invention, which is economical to carry out, makes it possible to produce cemented carbides having a unique combination of hardness and transverse rupture strength. lt was found by surprise that cemented carbides produced from tungsten carbide made of tungsten powder obtained by the hydrogen reduction of gaseous tungsten chloride, for example, as described in the aforementioned U.S. Pat. application Ser. No. 35,991, exhibit a highly homogeneous microstructure and good mechanical properties, such as high hardness in combination with high transverse rupture strength. The process described in the U.S. patent application for the hydrogen gas reduction of tungsten chloride offers remarkable technical and economical advantages over other reduction processes. One important advantage is the narrow grain size distribution which is obtainable. Another advantage is that the grain size can be varied within the industrially important range of less than 0.1 micron to several microns (e.g. up to 2 or 3 microns). The cemented carbide produced from such powders moreover, has proved to be highly insensitive to variations in the sintering temperature. For example, within the range of about l,300 to l,500C, it does not appear to show any or very little grain growth and, more over, the highly homogeneous microstructure as well as good mechanical properties are maintained. This implies a reduction of the production costs, because a relatively simple furnace construction can be used, or a greater volume of the available furnace equipment can be utilized. It was also found that the tungsten carbide produced via the reduced chloride powder is highly insensitive to grinding effects, which means that the microstructure of the cemented carbide can be determined with a fairly high accuracy in accordance with the grain size of the tungsten carbide.

DETAILS OF THE INVENTION The invention is described in greater detail as follows.

Tungsten carbide powders prepared from tungsten powder produced from hydrogen non-plasma reduced gaseous tungsten chloride (e.g. as in U.S. application Ser. No. 35,991) have very narrow grain size distribution as compared with conventional tungsten carbide powders produced from tungsten reduced from tungsten trioxide. With the invention, particularly narrow grain size distributions are obtained when fine-grained chloride tungsten powder of less than about 1 micron, especially less than about 0.5 micron, is carburized at above l,400C in hydrogen gas (unless stated otherwise, the grain size is measured by an electron microscope). When the new tungsten carbide powder is mixed with cobalt for sintering to produce cemented carbides, a system of high thermodynamic stability is obtained. The system does not exhibit any or very little grain growth. This is due to the fact that because of the narrow particle size range, there are very little or no fine grains which dissolve and precipitate on coarser grains as is the case with conventional tungsten carbide powder having a wide particle size distribution. Sintering tests at l,300l,500C have clearly shown that conventionally produced tungsten carbide powder tends to cause a substantial grain growth at increased temperature, whereas, the new tungsten carbide powder produced from chloride tungsten powder does not show substantially any grain growth, not even at increase sintering temperature.

The chloride tungsten powder is advantageous in that it can be provided with very high purity on an industrial scale. Consequently, substantially no voidforming reactions occur during the cemented carbide sintering. As stated above, cemented carbides produced from chloride tungsten powder, furthermore, show very good mechanical properties, such as high hardness combined with high transverse rupture strength. For a cemented carbide based on tungsten carbide with 13 percent cobalt as a binding phase, transverse rupture strength values of over 350 kg/mm with a hardness of 1,400 kg/mm have been obtained. These values are quite high when compared with transverse rupture strength values of about 300 kg/mm and hardness values of about l,l50 kg/mm obtained with conventional cemented carbide based on the same amount of carbide and binding phase and with substantially the same average grain sizes. Generally, it can be said that, depending on grain size, an increase in transverse rupture strength and hardness by 10 to 25 per cent over conventional cemented carbides can be obtained. It has also been found unexpectedly that a high transverse rupture strength of over 350 kg/mm in combination with a high hardness of over 1,400 kg/mm is obtainable from cemented carbides having a very fine grain size structure, for example, where the WC grains are less than 1 micron, and especially less than 0.5 micron. It goes without saying that such cemented carbides have advantages in the turning of special high speed steels or the cutting of other hard construction materials.

The invention is superior even to conventional cemented carbides produced by re-carburization. While in certain cases re-carburized cemented carbides may exhibit good transverse rupture strength values, hardness values have not been obtained as high as those obtained for cemented carbides produced according to the invention combined with high transverse rupture strength values. The difference in hardness amounts to abut 10 to 25 per cent and is of fundamental importance for cemented carbides used where resistance to wear is important, such as, for example, turning of metals. A factor of still greater importance is the reduction in production costs made possible by the present invention.

As illustrative of one method of providing the tungsten powder for subsequent carburizing to tungsten carbide the following is given:

A flow of 22 kg of WCl gas per hour is mixed with a flow of 96 liters of H per minute (referred to room temperature) in a nickel tube coupled to the chamber of a reactor heated to l,000C. The WCl gas had a temperature of 400C and the H gas was 525C. The mixture in the nickel tube prior to entry into the reactor had a temperature of about 440C, taking into account the heat transfer to the ambient environment. The gas velocity into the reactor was about 25 M/sec. The tungsten powder produced in the reactor had a residual chlorine content of about 0.26 percent by weight, a grain size of about 0.2 micron as determined by means of an electron microscope and an apparent density of about 1.32.

Thus, by providing the gaseous mixture in the nickel tube in the substantially unreacted state at a preheat temperature of above the reaction temperature of the mixture, for example, at a temperature of about 440C as against the approximate reaction temperature of the reactants of about 300 to 330C, the tungsten chloride is spontaneously reduced to tungsten powder as it enters the reactor to provide a narrow particle size range. By having a highly homogeneous mixture in which the reactants are at a predetermined ratio, temperature and velocity, the desired particle size is obtained. As a preferred method of producing the tungsten powder, reference is made to the aforementioned copending application Ser. No. 35,991, filed May 11, 1970, the disclosure of which is incorporated herein by reference in its entirety.

EXAMPLE 1 A batch of 5 kg of chloride tungsten powder produced similarly as described above with a grain size of about 0.23 micron BET was mixed with 6.27 percent by weight of carbon and carburized in hydrogen gas at 1,650C for 1 ,5 hours. The resulting tungsten carbide with a size of about 1.2 micron (BET) grain size (2.2 micron Fisher) was ground in a ball mill for 35 hours together together with 13 percent cobalt and 2 percent wax. The powder mixture was pressed at about 6 to 7 T.S.l. (tons per square inch) to provide bending test bars (6 mm high and 4 mm wide held on supports mm apart), and presintered at 950C for 1 hour in hydrogen gas atmosphere. The sintering was carried out in vacuum for 1 hours at l,420C. For comparison purposes, test bars of sintered cemented carbide produced from conventional tungsten carbide were also provided having substantially the identical grain size. The test bars were examined with respect to microstructure (1,500 x), porosity (ASTM B 276- 54), hardness and transverse rupture strength. The following results were obtained:

As will be clearly apparent, the cemented carbide produced in accordance with the invention exhibits high hardness combined with high transverse rupture strength, the hardness and bending strength being about 12 and 26 percent greater, respectively, than the same properties obtained for the conventionally produced material.

EXAMPLE 2 1n studying the effect of the sintering temperature on the microstructure and mechanical properties of the cemented carbide, a test was made on the basis of 5 kg of a tungsten carbide produced from chloride tungsten powder, with 1.8 micron (Fisher) particle size. The carbides were each ground in a ball mill for 35 hours together with 13 percent cobalt and 2 percent wax and test bars thereafter produced by pressing at about 6m 7 tons per square inch (T.S.I.). The presintering was carried out at 950C for 1 hour in hydrogen gas, and then followed by sintering in vacuum for 1 ,5 hours at the respective temperatures of 1,350C, 1,400C and 1,460C. The test were made in accordance with those stated for Example 1. The following results were obtained.

EXAMPLE 3 In order to study the properties of a finely ground cemented carbide, a finely ground tungsten powder (0.2 microns) was produced. The powder was carburized at 1,450C in a known manner in a carbon tube furnace. The grain size of the carbide was 0.4 microns. About 5 kgs of the carbide powder were milled in a ball mill for 35 hours together with 13 percent cobalt and 2 percent wax. The presintering was carried out at 950C for 1 hour under hydrogen and the sintering thereafter carried out under vacuum for 1 A hours at 1,3 C. The tests were made in accordance with the description in This example shows that according to the invention, very uniform grains can be obtained very simply and economically of high quality comparable to products obtained by much more complicated and expensive processes.

As will be noted, markedly superior results are obtained with the invention over the sintering range of about 1,350C to 1,460C.

Cemented carbide systems to which the invention is applicable include those in which the binder or matrix metal comprises a metal selected from the group consisting of the iron group metals iron, nickel and cobalt, in the amounts ranging from about 2 to 30 percent by weight and the balance essentially tungsten carbide. Cobalt is advantageously preferred as the binder metal and it is not uncommon to have tungsten carbide compositions containing about 2.5 to 6.5 percent by weight cobalt and the balance essentially tungsten carbide, or about 6.5 to 15 percent by weight cobalt and the balance essentially tungsten carbide, or even about 15 to 30 percent by weight cobalt and the balance essentially tungsten carbide. it is understood that certain amounts of other carbides, such as TiC and/or TaC may be present in the composition and the term balance essentially allows for the presence of such carbides.

Typical examples of tungsten carbide-cobalt compositions are as follows:

% Cobalt Tungsten Carbide 5 balance essentially WC l balance essentially WC l balance essentially WC balance essentially WC balance essentially WC balance essentially WC which comprises,

providing tungsten powder of substantially uniform grain size produced from a gaseous mixture of the reactants tungsten chloride and hydrogen pre heated at a temperature above the reaction temperature of said reactants, which preheated gaseous mixture is then subjected to substantially spontaneous reduction in a reaction chamber maintained at above the reaction temperature,

carburizing said powder to produce tungsten carbide,

and sintering a compacted mixture of said tungsten carbide and a binder metal selected from the group consisting of iron, nickel, cobalt and alloys of these metals with each other,

whereby to produce a sintered product characterized by said improved combination of physical properties.

2. The process of claim 1, wherein the amount of binder metal ranges from about 2 to 30 percent by weight of the composition.

3. The process of claim 2, wherein the binder metal is cobalt.

4. The process of claim 3, wherein the amount of cobalt binder metal ranges from about 2.5 to 6.5 percent by weight.

5. The process of claim 3, wherein the amount of cobalt binder metal ranges from about 6.5 to 15 percent bly weight. x

6. he process of claim 3, wherein the amount of cobalt binder metal ranges from about 15 to 30 percent by weight.

7. The cemented carbide product produced in accordance with the process of claim 2, characterized by an improved combination of the physical properties bending strength and hardness. 

1. A process of producing a cemented carbide product characterized by an improved combination of the physical properties bending strength and hardness which comprises, providing tungsten powder of substantially uniform grain size produced from a gaseous mixture of the reactants tungsten chloride and hydrogen preheated at a temperature above the reaction temperature of said reactants, which preheated gaseous mixture is then subjected to substantially spontaneous reduction in a reaction chamber maintained at above the reaction temperature, carburizing said powder to produce tungsten carbide, and sintering a compacted mixture of said tungsten carbide and a binder metal selected from the group consisting of iron, nickel, cobalt and alloys of these metals with each other, whereby to produce a sintered product characterized by said improved combination of physical properties.
 2. The process of claim 1, wherein the amount of binder metal ranges from about 2 to 30 percent by weight of the composition.
 3. The process of claim 2, wherein the binder metal is cobalt.
 4. The process of claim 3, wherein the amount of cobalt binder metal ranges from about 2.5 to 6.5 percent by weight.
 5. The process of claim 3, wherein the amount of cobalt binder metal ranges from about 6.5 to 15 percent by weight.
 6. The process of claim 3, wherein the amount of cobalt binder metal ranges from about 15 to 30 percent by weight. 